A certain amount of entrained air is necessary for improving the freeze-thaw durability of concrete, which is a brittle material. However, the nature and extent of air entrainment must be carefully controlled because excessively large or unevenly spaced air voids within the concrete can lead to loss of freeze-thaw durability and compressive strength. Air entraining agents such as rosins and anionic surfactants are commonly used to control the size and spacing of air voids within the cementitious mix.
In European Patent EP 0 415 799 B1 (owned by the common assignee hereof), Gartner et al. taught that additions to cementitious compositions of a higher alkanolamine, such as triisopropanolamine (“TIPA”), enhanced late strength (e.g., at 28 days) but also increased the amount of air entrained in the cementitious composition. Analysis of various cement samples revealed an increase in air entrainment of about two percent when compared to cement that did not contain TIPA. Gartner et al. suggested using air-detraining nonionic surfactants, which included phosphates (e.g., tributylphosphate), phthalates (e.g., diisodecylphthalate), and polyoxypropylene-polyoxyethylene block copolymers. See EP 0 415 799 B1 at Page 6, lines 40-53.
In U.S. Pat. No. 5,156,679 (owned by the common assignee hereof), Gartner et al. taught the use of water-soluble alkylated alkanolamine salts for detraining air in concrete. These materials included N-alkylalkanolamine and N-alkyl-hydroxylamine. When TIPA was added to a mortar mix in the amount of 0.02% by weight as part of the water of hydration along with 0.01% by weight of dibutylamino-2-butanol (“DBAB”) as a defoamer, the mortar mix demonstrated a reduction in air entrainment (Col. 5, line 51-Col. 6, line 14).
When concrete is formed, it requires mixing of the various components such as hydraulic cement, sand, gravel, water, and possibly additives to form a substantially uniform mixture. During mixing, air becomes entrapped in the composition, and much of this air remains in the resultant cured composition in the form of air voids. If air void size is small, the mix is said to be “air entrained.” In most instances, a small amount of air entrainment is tolerated, and, in other instances, this is desirable for enhancing durability of the concrete in terms of resisting freeze/thaw cycles.
However, excessive air entrainment in the hydraulic cement composition will cause the resultant structure to have lower compressive strength than the mixture design is otherwise capable of attaining. There is an inverse relationship between air entrainment and compressive strength. It is generally believed that for each volume percent of air voids (bubbles) in a concrete mass, there exists a five percent loss of compressive strength. Hence, management of the air void content and nature is vitally important.
Water-reducing admixtures (including superplasticizers) are particularly known to entrain excessive air in the concrete slurry. The industry has attempted to correct this by using air-detraining agents (otherwise called defoamers herein) in the concrete mix. As with the cement grinding additives (e.g., TIPA) previously mentioned, common admixtures include hydrophobic materials such as nonionic surfactants with low hydrophilic-lipophilic balance (HLB), silicone derivatives, tributylphosphate, and alkyl phthalates.
However, defoamers with high hydrophobic properties have, as a consequence, very limited solubility in water, and are not easily incorporated into the aqueous solutions which make up the water-reducing admixtures. Their hydrophobic nature tends to destabilize the aqueous product by fostering separation of the components, and requires that the water-reducing admixture and defoamer be stored in separate tanks and mixed immediately before use; or, alternatively, that they be pre-mixed and constantly stirred to prevent separation.
In U.S. Pat. Nos. 5,665,158 and 5,725,657 (owned by the common assignee hereof), Darwin et al. disclosed the use of oxyalkylene amine-based defoaming agents for use with polycarboxylate superplasticizers. An alkoxypolyoxyalkylene ammonium polymer was ionically attached to the carboxylate portion of the comb polymer backbone to impart desired air controlling properties to the hydratable concrete being treated.
In U.S. Pat. No. 6,139,623 (owned by the common assignee hereof), Darwin et al. disclosed polycarboxylate superplasticizers emulsified with antifoaming agent using a surfactant to stabilize the emulsified superplasticizer and antifoaming agent. The antifoaming agent could be selected from phosphate esters (e.g., dibutylphosphate, tributylphosphate), borate esters, silicone derivates (e.g., polyalkyl siloxanes), and polyoxyalkylenes having defoaming properties.
In U.S. Pat. No. 6,858,661 (owned by the common assignee hereof), Zhang et al. disclosed a polycarboxylate water-reducer and a tertiary amine defoamer having an average molecular weight of 100-1500 for creating a stable admixture formulation and helping to achieve a controllable level of entrained air in the concrete mix.
In U.S. Pat. No. 6,545,067 (owned by BASF), Buchner et al. disclosed mixtures of polycarboxylate superplasticizer and butoxylated polyalkylene polyamine as a defoamer for reducing the air pore content of cement mixes.
In U.S. Pat. No. 6,803,396 (also owned by BASF), Gopalkrishnan et al. disclosed mixtures of polycarboxylate superplasticizer and air-detraining agents. The air detrainers were based on low molecular weight block polyether polymers described as containing ethylene oxide and propylene oxide units and described as being initiated using reactive diamine or glycol groups.
In U.S. Pat. No. 6,569,924 (owned by MBT Holding AG), Shendy et al. disclosed the use of polycarboxylate dispersants, a water-insoluble defoamer, and a solubilizing agents for solubilizing the water-insoluble defoamer. Such solubilizing agents functioned by increasing the amount of oil component within the aqueous phase. A similar approach was taken in U.S. Pat. No. 6,875,801 wherein Shendy et al. described using amine solubilizing agents for stabilizing water-insoluble defoamers.
Regardless of whether the defoamer is grafted onto a polymer dispersant or emulsified or rendered more water-soluble within the additive composition, the present inventor believes that a critical problem still remains with avoiding phase separation within the water-based additive formulation while retaining the efficacy of the defoamer to detrain air within the cementitious mixture being treated.
Accordingly, an improved defoamer additive composition is needed for improving the stability of air-entraining additives used in cement or concrete mixes without curtailing the effectiveness of the defoamer as an air-detraining agent. The present inventor believes that a new class of defoamers for use with air-entraining agents, such as higher trialkanolamines and water-reducing agents, is needed for controlling air content in cementitious materials, and also for providing different degrees of defoaming power, yet with increasing water compatability that provides resistance to phase separation.